I. Field of the Invention
The present invention relates generally to antennas for wireless devices, and more specifically, to a internally mounted antennas. The invention further relates to internal substrate antennas for wireless devices, with parasitic elements having improved energy coupling characteristics, and gain and bandwidth for the wireless devices.
II. Description of the Related Art
Antennas are an important component of wireless communication devices and systems. Although antennas are available in numerous different shapes and sizes, they each operate according to the same basic electromagnetic principles. An antenna is a structure associated with a region of transition between a guided wave and a free-space wave, or vice versa. As a general principle, a guided wave traveling along a transmission line which opens out will radiate as a free-space wave, also known as an electromagnetic wave.
In recent years, with an increase in use of personal wireless communication devices, such as hand-held and mobile cellular and personal communication services (PCS) phones, the need for suitable small antennas for such communication devices has increased. Recent developments in integrated circuits and battery technology have enabled the size and weight of such communication devices to be reduced drastically over the past several years. One area in which a reduction in size is still desired is communication device antennas. This is due to the fact that the size of the antenna can play an important role in decreasing the size of the device. In addition, the antenna size and shape impacts device aesthetics and manufacturing costs.
One important factor to consider in designing antennas for wireless communication devices is the antenna radiation pattern. In a typical application, the communication device must be able to communicate with another such device or a base station, hub, or satellite which can be located in any number of directions from the device. Consequently, it is essential that the antennas for such wireless communication devices have an approximately omnidirectional radiation pattern, or a pattern that extends upward from a local horizon.
Another important factor to be considered in designing antennas for wireless communication devices is the antenna's bandwidth. For example, wireless devices such as phones used with PCS communication systems operate over a frequency band of 1.85-1.99 GHz, thus requiring a useful bandwidth of 7.29 percent. A phone for use with typical cellular communication systems operates over a frequency band of 824-894 MHz, which requires a bandwidth of 8.14 percent. Accordingly, antennas for use on these types of wireless communication devices must be designed to meet the appropriate bandwidth requirements, or communication signals are severely attenuated.
One type of antenna commonly used in wireless communication devices is the whip antenna, which is easily retracted into the device when not in use. There are, however, several disadvantages associated with the whip antenna. Often, the whip antenna is subject to damage by catching on objects, people, or surfaces when extended for use, or even when retracted. Even when the whip antenna is designed to be retractable in order to minimize such damage, it can still require a minimum device housing dimension when retracted that is larger than desired.
Whip antennas are often used in conjunction with short helical antennas which are activated when the whip is retracted into the phone. The helical antenna provides the same radiator length in a more compact space to maintain appropriate radiation coupling characteristics. While the helical antenna is much shorter, it still protrudes a substantial distance from the surface of the wireless device impacting aesthetics and catching on other objects. To position such an antenna internal to the wireless device would require a substantial volume, which is undesirable. In addition, such helical antennas seem to be very sensitive to hand loading by wireless device users.
Another type of antenna which might appear suitable for use in wireless communication devices is a microstrip or stripline antenna. However, such antennas suffer from several drawbacks. They tend to be much larger than desired, suffer from lower bandwidth, and lack desirable omnidirectional radiation patterns.
As the term suggests, a microstrip antenna includes a patch or a microstrip element, which is also commonly referred to as a radiator patch. The length of the microstrip element is set in relation to the wavelength .lambda..sub.0 associated with a resonant frequency f.sub.0, which is selected to match the frequency of interest, such as 800 MHz or 1900 MHz. Commonly used lengths of microstrip elements are half wavelength (.lambda..sub.0 /2) and quarter wavelength (.lambda..sub.0 /4). Although, a few types of microstrip antennas have recently been used in wireless communication devices, further improvement is desired in several areas. One such area in which a further improvement is desired is a reduction in overall size. Another area in which significant improvement is required is in bandwidth. Current patch or microstrip antenna designs do not appear to obtain the desired 7.29 to 8.14 percent or more bandwidth characteristics desired for use in most communication systems, in a practical size.
Conventional patch and strip antennas have further problems when placed near the extensive ground planes found within most wireless devices. The ground planes can alter the resonant frequency, creating a non-repeatable manufactured design. The minimum surface area also prevents mounting in a fashion that optimizes the radiation patterns. In addition, "hand loading", that is, placement of a user's hand near the antenna dramatically shifts the resonant frequency and performance of the antenna.
Radiation patterns are extremely important not only for establishing a communication link as discussed above, but also in relation to government radiation standards for wireless device users. The radiation patterns must be controlled or adjusted so that a minimum amount of radiation can be absorbed by device users. There are governmental standards established for the amount of radiation that can be allowed near the wireless device user. One impact of these regulations is that internal antennas cannot be positioned in many locations within a wireless device because of theoretical radiation exposure for the user. However, as stated above, when using current antennas in other locations, ground planes and other structures often interfere with their effective use.
With the above problems in mind a new type of antenna referred to as a substrate antenna has been developed to provide an internal antenna for wireless devices having appropriate bandwidth characteristics along with reduced size, adequate gain, and reduced response to or impact from hand loading, or similar problems encountered within the art. This type of antenna is disclosed in copending U.S. patent application Ser. No. 09/028,510 entitled "Substrate Antenna" filed on Feb. 23, 1998, which is incorporated herein by reference.
Although the substrate antenna advances the art of internal antennas and solves several problems in the art, there are some situations in which the antenna does not achieve desired gain or energy distribution conditions. That is, the antenna directs or couples radiation into undesired modes or directions, reducing the antenna gain. In addition, substrate and other types of small internal antennas are also negatively impacted by being positioned adjacent to various noise sources within the wireless device. When placed inside a wireless device the antenna may be positioned relatively close to conductors used to transfer signals or power. Antenna gain and wireless device sensitivity can decrease due to signals or signal noise being coupled into the antenna from these conductors or various sources within the wireless device.
Therefore, a new antenna structure and technique for manufacturing and mounting antennas within wireless devices is needed to achieve internal antennas having desired gain and sensitivity or reduced noise characteristics.